Various types of liquid coating compositions have been applied to metallic substrates and baked thereon in order to protect the substrates against corrosion. Certain of such coatings are applied in conventional metal coil coating processes, and they must be sufficiently adherent and flexible to resist cracking, chipping and peeling. One process which has been utilized for improving the corrosion-resistance of various metal substrates generally involves the application of two coatings. The first coating is comprised of a material such as xanthan gum as carrier for the other ingredients of the coating which include a chromium compound such as chromium trioxide and zinc dust. On baking, the xanthan gum contained in the coating becomes water-insoluble. Generally, baked temperatures of at least about 500.degree. F. are required. Over this baked coating is applied a second coating which comprises a zinc rich resin. U.S. Pat. No. 4,026,710 (Kennedy) describes such two-step procedures for improving the corrosion-resistance of metal.
U.S. Pat. No. 3,713,904 (Bernath et al) describes compositions and methods for producing corrosion-resistant and protective coatings on aluminum and aluminum alloys. The coating on the metal substrate comprises an organic resin, an inorganic hexavalent chromium compound, an oxidizable component, phosphoric acid and strontium chromate. On mixing, the strontium chromate and oxidizable component react to reduce the hexavalent chromium to trivalent chromium. The mixture is applied to the substrate which is then heated at an atmosphere of from about 600.degree. F. to about 800.degree. F. to achieve a metal temperature of at least about 450.degree. F. which results in the oxidation of a portion of trivalent chromium to hexavalent chromium resulting in a strongly adherent organic resin coating. The organic resins described include epoxy resins. The patentees also describe the application of various top coats including vinyl top coats and finish coats comprising strontium chromate--potassium dichromate containing fluorocarbon finish coats.
U.S. Pat. No. 4,352,899 (Tada et al) describes a coating composition for metal substrates which comprises an epoxy resin, an organic phosphorus compound, zinc powder, and a magnesium compound dispersed in a liquid medium. A portion of the zinc powder can be replaced by a solid electroconductive material. Compositions are applied in organic solvents, and the compositions may contain various pigments including chromate pigments.
U.S. Pat. No. 4,381,334 (Balk et al) describes zinc-rich electrostatic powders comprising zinc incorporated into an epoxy resin powder. A variety of solid epoxy resins are described as being useful. The compositions may also contain a curing agent and other additives such as flow modifiers.
U.S. Pat. No. 4,461,857 (Sekmakas et al) describes thermosetting aqueous coating compositions having improved properties when the coating compositions are modified to include a water-dispersible epoxy phosphate. The epoxy phosphate-solvent mixtures are described in U.S. Pat. No. 4,598,109.
Zinc-rich coatings useful for improving corrosion-resistance are described in U.S. Pat. No. 4,476,260 (Salensky). The coatings comprise a mixture of zinc pigment, thermoplastic epoxy resins, an organosilane and optionally aluminum trihydrate and one or more dispersing agents. A washcoat composition suitable for application to tin-plated mild steel is described in U.S. Pat. No. 4,544,686 (Bromley et al), and the composition consists of an aqueous carrier medium and a binder comprising a thermosetting acrylic polymer, an epoxy resin, an acid catalyst which may be phosphoric acid or chromic acid or an ammonium or amine salt thereof. Epoxy resin-containing coatings also are described in U.S. Pat. No. 4,544,213, (Marini et al) where the patentees describe coatings of a mixture of phosphates and resins. The resins may be acrylic base resins, ethylene-vinyl acetate copolymer resins, or thermosetting epoxy resins.
Based upon English language abstracts of Japanese Patent Applications Nos. 59052645A and 59035934A, these patents describe a weldable coated steel sheet which has been obtained by applying a first coating comprising chromium trioxide, about 50% of which has been reduced to the trivalent state, phosphoric acid, poly-acrylic acid, and acrylic emulsion in water. The coated metal is then roll-coated with zinc-manganese rich coatings which contain resin such as epoxy resins. The coating then is baked at about 260.degree. C. for one minute.
Japanese Patent Application No. 4936734 describes coating a metal sheet with a corrosion-inhibiting agent of an epoxy-polyamide resin diluted in an organic solvent and mixed with phosphoric acid and chromium phosphate. The coating deposited by this solution is then top-coated with a mixture of Portland cement, and ethylene-vinyl acetate copolymer in water.